The term "voter turnout" refers to the portion of

citizens who register to vote before an election

eligible voters who cast a ballot in an election

citizens who choose not to register to vote

eligible voters who choose not to vote

The term "voter turnout" refers to eligible voters who cast a ballot in an election. So, a turnout refers to the number of people who 'turned out,' who appeared somewhere to do something. In this case, it refers to the number of registered voters who went to vote on presidential, or whatever other elections there are.

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